1-Butene, as well as other C4 hydrocarbons (such as isobutene and 2-butenes), is obtained in large amounts from technical C4 cuts, for example the C4 cut from steamcrackers or FCC units. These C4 cuts comprise butadiene, the monoolefins isobutene and 1-butene, the two 2-butenes, and also the saturated hydrocarbons isobutane and n-butane. Owing to the small boiling point differences of the ingredients and their low separation factors, a distillative workup is difficult and uneconomic. Linear butenes are therefore usually obtained from other products by a combination of chemical reactions and physical separating operations.
The first step in obtaining linear butenes, which is common to all workup variants, when butadiene is present, is removal of most of the butadiene. The butadiene is either removed by an extractive distillation or chemically converted to butanes by selective hydrogenation. What remains, after either the extractive distillation or chemical conversion of butadiene, is a hydrocarbon mixture (so-called raffinate I or hydrogenated crack-C4). The raffinate I comprises the saturated hydrocarbons n-butane and isobutane, the olefins isobutene and 1-butene, and cis and trans 2-butenes. Polyunsaturated C4 hydrocarbons are present typically, in the raffinate I, in a fraction below 1%.
Because the boiling points of 1-butene and isobutene are very close together, it is not possible to remove 1-butene from the raffinate I, in an economically viable manner, by a simple distillation. Isobutene is therefore very substantially removed from the raffinate I by a selective chemical reaction. After the removal of the isobutene, a hydrocarbon mixture (raffinate II) remains. The raffinate II comprises linear butenes and the saturated hydrocarbons isobutane and n-butane. The components of raffinate II can be further separated by distillation, for example into isobutane and 1-butene, and a mixture of the two 2-butenes and n-butane. In further distillation steps, 1-butene which contains only small amounts of isobutene can be obtained in high purity from the 1-butenic fraction. Highly pure 1-butene is desirable because 1-butene is used to a large degree as a comonomer in ethylene polymerization, where isobutene impurities are undesired. Typical specifications of 1-butene therefore restrict the content of isobutene in the 1-butene to below 2000 ppm.
For the selective chemical reaction of the isobutene, which substantially removes isobutene from the raffinate I, various processes are known. One way of removing isobutene is to react the isobutene with alcohols, for example methanol or ethanol, to give the corresponding tertiary butyl ethers. The advantage of this reaction is that the isobutene can be converted virtually fully with high selectivity in the presence of linear butenes (without noticeable conversion of n-butenes occurring). For this purpose, various process technology variants have been developed. The technique of reactive distillation has been found to be particularly useful for achieving high isobutene conversions.
The industrially most significant process is the reaction of isobutene with methanol to give methyl tert-butyl ether (MTBE) which finds a great degree of use mainly as a fuel additive.
A further means of chemical conversion of the isobutene is the reaction with water to give tert-butyl alcohol (TBA). Owing to the low solubility of water in C4 hydrocarbons, this route is technically more complex than the ether syntheses.
Another possibility for removing isobutene is to oligomerize the isobutene and remove the oligomerizate. A disadvantage of oligermizing isobutene is that a large portion of the linear butenes present are also, and undesirably, converted to cooligomers or homooligomers during the isobutene oligomerization. A further disadvantage is the partial isomerization of 1-butene to the cis and trans 2-butenes.
A further means of to remove isobutene, in the presence of other C4 hydrocarbons, is to react isobuten with formaldehyde. The products obtained are processed further, to give, for example, isoprene.
Many of the known conversions of isobutene, for example the conversion to tert-butyl alcohol (TBA) or the conversion to isobutene oligomers, do not afford full isobutene conversion or, in the alternative, afford only poor selectivities, at high conversions, in the presence of linear butenes. An example of a solution proposed is a combination of these processes with a simultaneous (EP 0 048 893, DE 29 44 457) or subsequent conversion of the remaining isobutene to tert-butyl ethers.
U.S. Pat. No. 4,797,133 describes, inter alia, a process wherein in a first reaction isobutene is removed from the starting hydrocarbon mixture, (for example by reaction to give tert-butyl alcohol (TBA)), and the remaining residue is then converted to an ether in an etherification reaction.
DE 103 02 457 describes a process for preparing butene oligomers and tert-butyl ethers from isobutenic C4 streams, in which the isobutene can be removed from a substantially butadiene-free C4 hydrocarbon stream with only small losses of linear butenes. In this process, a portion of the isobutene is oligomerized, using an acid catalyst, in a first reaction step and then the remaining isobutene is removed in a second reaction step by reacting the remaining isobutene with alcohol to give a tert-butyl ether. The etherification reaction takes place in a reactive distillation column.
DE 25 21 964 describes a two-stage process for preparing alkyl tert-butyl ethers. In the first stage, isobutene in the reaction mixture is reacted with an alcohol to form an ether, and the resulting ether is then removed from the reaction mixture. The residue remaining after removal of the ether is conducted into a second reaction stage for the conversion of the remaining isobutene.
All processes which comprise a partial conversion of the isobutene followed by conversion of the remaining isobutene fraction from within the entirety of the C4 hydrocarbons suffer from two disadvantages. Firstly, the large amounts of material that have to be conducted into the second reaction step, and secondly, the relatively low concentrations of isobutene in the mixture. Both disadvantages generally force the apparatus equipment to be of undesirably large size and also, in most cases, result in increased energy consumption.